U.S. Pat. Nos. 3,934,917 and 4,174,131, both of which are assigned to the assignee of the present invention, disclose single hoist line operated clamshell buckets for handling bulk materials. Such buckets are automatically operated open or closed by either the presence or absence of tension in the single hoist line for the bucket. Bucket loading with the bulk material automatically occurs when the hoist line is tightened with the buckets supported on the bulk material. To discharge a full bucket, it was only necessary to lower the bucket until the bucket weight was removed from the hoist line in order that the hoist line would slack off sufficiently for the mechanical closing latch to release. The necessity to lower the bucket onto a support surface to remove the weight from the buckets and hoist line to discharge the buckets was a time consuming step that significantly increased cargo transfer time. Another method involved manual tripping in the air of the bucket release mechanism by means of a rope or line attachment. This operation involved additional personnel, risk of injury and a clumsy operational sequence.
The mechanically operated latching mechanism was also subject to fatigue failure from the repeated latching and releasing operation of the buckets. A number of improvements to overcome the problems associated with the wear of or damage to the buckets latch and release mechanisms have been made. One such improvement was disclosed in U.S. Pat. No. 4,043,580 which employed a remotely actuated hydraulic operating cylinder in conjunction with the mechanical latching mechanism. In one disclosed embodiment a radio controlled valve is used in conjunction with a limit switch to control discharge of the clamshell bucket.